A sentry is someone who stands guard or watches against some intrusion or unwelcome activity. Your dog stands sentry over your house, but he would be more effective if he barked at strangers instead of licking their hands.
The noun sentry came from the French sentinelle, which has a similar meaning. It was in the 17th Century that the word was first associated with someone on a watchtower as a military guard. You could only enter the dreaded ice fortress after sneaking past the sentry posted outside. Don't let his button eyes and carrot nose fool you — he's a sharp one!